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Reactive Power Document and

Voltage Control of North Eastern Region

December-2017

Edition-9

North Eastern Regional Load Despatch Centre

Shillong Power System operation Corporation Limited

(A Government of India Enterprise)

A Typical SVC Station

REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION

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CONTENTS

CONTENTS .......................................................................................................................................................................1

List Details. ........................................................................................................................................................................2

List of Figures: ...................................................................................................................................................................2

List of Tables: ....................................................................................................................................................................3

1 REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL .....................................................................6

1.1 Introduction ................................................................................................................................ 6

1.2 Analogy of Reactive Power ................................................................................................... 8

1.3 Understanding Vectorally .................................................................................................... 10

1.4 Voltage Stability .................................................................................................................. 11

1.5 Voltage Collapse .................................................................................................................. 12

1.6 Proximity to Instability ....................................................................................................... 14

1.7 Reactive Reserve Margin .................................................................................................... 15

1.8 NER GRID – Overview ............................................................................................................. 18

1.9 Reliability Improvement Due to Local Voltage Regulation .............................................. 21

2 TRANSMISSION LINES AND REACTIVE POWER COMPENSATION ................................................................. 22

2.1 Introduction ........................................................................................................................... 22

2.2 Surge Impedance Loading (SIL) ........................................................................................... 23

2.3 Shunt Compensation in Line ............................................................................................. 23

2.4 Line loading as function of Line Length and Compensation ........................................... 24

3 SERIES AND SHUNT CAPACITOR VOLTAGE CONTROL ...................................................................................... 30

3.1 Introduction ...................................................................................................................... 30

3.2 MeSeb Capacity Building And Training Document Suggest (Sub Title As Given In The PFC

Document For Corporatization Of MeSeb): ................................................................................... 31

3.3 As Per The Assam Gazette, Extraordinary, February 10, 2005 ......................................... 31

4 TRANSFORMER LOAD TAP CHANGER AND VOLTAGE CONTROL ...................................................................... 33

4.1 Introduction ..............................................................................................................................33

4.2 As Per The Assam Gazette, Extraordinary, February 10, 2005 .............................................. 35

5 HVDC AND VOLTAGE CONTROL ........................................................................................................................... 37

5.1 Introduction .......................................................................................................................... 37

5.2 HVDC Configuration ............................................................................................................ 37

5.3 Reactive Power Source ...................................................................................................... 40

5.4 ±800 kV HVDC Bi-Pole ....................................................................................................... 40

5.5 Technical details of Biswanath Chariali –Alipurduar-Agra HVDC: ........................................ 41

5.6 Impact of Largest Filter Switching Under Different HVDC Power Order. ........................... 43

6 FACTS AND VOLTAGE CONTROL ..................................................................................................................... 44

6.1 Introduction ...................................................................................................................... 44

6.2 Static Var Compensator (SVC) ............................................................................................... 44


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6.3 Converter-based Compensator ............................................................................................. 45

6.4 Series-connected controllers ............................................................................................... 46

7 GENERATOR REACTIVE POWER AND VOLTAGE CONTROL ......................................................................... 47

7.1 Introduction .......................................................................................................................... 47

7.2 Synchronous Condensers .................................................................................................... 49

8 CONCLUSION ........................................................................................................................................................ 50

9 SUMMARY .............................................................................................................................................................. 51

10 STATUTORY PROVISIONS FOR REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL .......... 54

10.1 Provision in the Central Electricity Authority (Technical Standard for connectivity to the

grid) Regulations 2007 [8]: ............................................................................................................ 54

10.2 Provision in The Indian Electricity Grid Code (IEGC), 2010: .......................................... 54

11. BIBLIOGRAPHY: ................................................................................................................................................. 59

List Details.

List 1 International connectivity of NER at 400kV (Charged at 132kV) ....................................... 25

List 2 International Connectivity of NER at 132kV ......................................................................... 25

List 3 +/- 800 kV HVDC Lines Agra-BNC ..................................................................................... 25

List 4: Fixed, Switchable and Convertible Line reactors in North Eastern Region ......................... 26

List 5: Bus Reactors in North Eastern Region .................................................................................. 28

List 6: List of Upcoming Bus Reactors in North Eastern Region……………………………………………. 29

List 7: Tertiary Reactors on 33kV side of 400/220/33 kV ICTs in North Eastern Region .............. 29

List 8: Shunt Capacitors details in North Eastern Region ............................................................... 32

List 9: Transmission/Transformation/VAR Compensation Capacity of North Eastern Region ... 36

List of Figures:

Figure 1 Voltage and Current Waveform ............................................................................................ 6

Figure 2 Power Triangle ....................................................................................................................... 7

Figure 3 Boat Pulled by Horse ............................................................................................................ 8

Figure 4 Direction of Pull .................................................................................................................... 8

Figure 5 Vector Representation of Analogy ........................................................................................ 8

Figure 6 Labyrint Spel ......................................................................................................................... 9

Figure 7 Vector Representation ......................................................................................................... 10

Figure 8 Time frames for voltage stability phenomena ..................................................................... 13

Figure 9 PV curve and Voltage stability margin under different conditions .................................... 14

Figure 10 Average cost of Reactive power technologies .................................................................... 17

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Figure 11 NER Grid map..................................................................................................................... 18

Figure 12 Switching principle of LTC .................................................................................................33

Figure 13: An example of voltage scatter plot…………………………………………………………………………35

Figure 14 HVDC Fundamental components ..................................................................................... 39

Figure 15: Schematic Diagram of HVDC-BNC ................................................................................... 41

Figure 16 Static VAR Compensators (SVC): TCR/TSR, TSC, FC and Mechanically Switched

Resistor ............................................................................................................................................... 45

Figure 17 STATCOM topologies:(a) STATCOM based on VSI and CSI (b) STATCOM with storage

............................................................................................................................................................ 45

Figure 18 Series-connected FACTS controllers: (a) TCSR and TSSR; (b) TSSC; (c) SSSC .............. 46

Figure 19 D-Curve of a typical Generator .......................................................................................... 47

List of Tables:

Table 1: Reactive power compensation sources ................................................................................. 16

Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type ................... 24

Table 3: Equipment preference ......................................................................................................... 30

Table 4: AC Filter Bank at HVDC Agra ............................................................................................. 42

Table 5: AC Filter Bank at HVDC BNC. ............................................................................................ 42

Table 6: Impact of Largest Filter Switching under different HVDC Power order. .......................... 43

Table 7: List of units in NER required to be normally operated with free governer action and AVR

in service ............................................................................................................................................ 49

Table 8: IEGC operating voltage range .............................................................................................. 56

ANNEXURES

Annexure I: Fault levels of major substation in NER

Annexure II: List of Lines in North Eastern Region

Annexure III: List of ICTs in North Eastern Region

Annexure IV: Substations in North Eastern Region

Annexure V: Capability curves of various generators in NER GRID

Annexure VI: The AEGCL Gazette, Extraordinary, February 10, 2005











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EXECUTIVE SUMMARY Quality of power to the stakeholders is the question of the hour worldwide. Enactment of

several regulations viz. IE act – 2003, ABT, Open access regulations, IEGC, DSM and

several other amendments are in the direction towards improvement of system reliability

and power quality.

It is also significant to mention that due to the massive load growth in the country, the

existing power networks are operated under greater stress with transmission lines

carrying power near their limits. Increase in the complexity of network and being loaded

non-uniformly has increased its vulnerability to grid disturbances due to abnormal

voltages (High and Low). In the past, reason for many a black outs across the world have

been attributed to this cause.

Three objectives dominate reactive power management. Firstly, maintaining adequate

voltage throughout the transmission system under normal and contingency conditions.

Secondly, minimizing congestion of real – power flows. Thirdly, minimizing real – power

losses. Also with dynamic ATCs, var compensation, congestion charges, if not seriously

thought, it may have serious commercial implications in times to come due to the amount

of bulk power transfer across the country.

Highlights of rolling year of NER grid include commercial operation of 400/220 kV, 315

MVA ICT 2 at BgTPP, 132 kV Pasighat- Roing S/C line and 132 kV Roing – Tezu S/C line.

BgTPP ICT 2 commissioning has fulfilled the N-I contingency requirement of existing

400/220 kV 315 MVA ICT at Bongaigaon. Commissioning of 132 kV Pasighat – Roing –

Teju has enabled Roing and Teju areas of Arunachal Pradesh to get connected with the all

India Grid. NTPC second unit of capacity 250 MW declared commercially operational in

the month of November’ 2017.

Other major elements commissioned during current year were 20 MVAR Line Reactor in

220 kV Mariani(PG)- AGBPP at AGBPP , 20 MVAR Bus reactor at Roing(PG), 20 MVAR

Bus reactor at Teju(PG), 132/33 kV, 3x5 MVA ICT I & II at Roing (PG), 132/33 kV, 3x5

MVA ICT I & II at Teju (PG) , 132 kV Doyang - Wokha which was further LILO at Sanis,

LILO of 132 kV Aizwal- Zuangtui at Melriat(PG) and 420 kV 63 MVAR Line Reactors (to

be used as Bus Reactor) connected to 400 kV Lower Subansiri – Biswanath Chariali – I

Line Bay & 400 kV Lower Subansiri – Biswanath Chariali - III Line Bay at Biswanath

Chariali (POWERGRID).


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Commissioning of Melriat substation has strengthened the connectivity of Mizoram with

NER grid. Wokha Substation commissioning has improved connectivity in Nagaland and

it has also provided another evacuation path for DHEP (Doyang Hydro-Electric Power

Project) of NEEPCO. With the commissioning of these elements state network

connectivity with NER grid has further been strengthened. With the increase in

controllability compared to earlier years, grid operation has been smooth and grid

parameters were maintained within the prescribed IEGC limits.

Even though there has been improvement in connectivity of NER grid, this year NER grid

experienced a unique event of sudden reduction in voltage to a very low level without loss

of connectivity with the grid, commonly known as “BROWNOUT”. The event occurred in

Tripura system at 16:11 Hrs on 23-Sep-2017. Before the incident AGTCCPP Power Station,

Tripura Power System and South Comilla (Bangladesh) Power System were connected

with rest of NER Grid through 400/132 kV, 125 MVA ICT- 2 at Palatana, 132 kV

AGTCCPP - Kumarghat I line and 132 kV P K Bari - Kumarghat line. 125 MVA ICT-I at

Palatana was under planned shutdown. After shutdown was withdrawn, while closing HV

side CB of ICT – I, B-phase CB of ICT-I failed to close due to unhealthy operating

mechanism resulted in unbalanced neutral current in ICT-II at Palatana. Before operation

of Pole Discrepancy relay of ICT-I HVCB, ICT-II tripped on Back up Earth fault

Protection. After tripping of both ICTs at Palatana, thus removing the reactive power

support being provided by Palatana generation, Voltage Collapse was observed in Tripura

Power System. Sharp decline in voltage observed at Agartala Bus, voltage went down to

around 5 kV. All state generation were de-synchronized due to low voltage from the Grid.

Although the voltage dropped down to 5 kV, Tripura Power System was still connected to

the rest of NER Grid, thus causing Brownout. Loads were manually disconnected to

improve voltage. This event of Brownout caused huge load and generation loss in Tripura

and South Comilla (Bangladesh). The event has raised the need of sufficient local reactive

power supports like capacitor banks in Tripura system.

This manual is in continuation to the previous edition for understanding the basics of

reactive power and its management towards voltage control, its significance and

consequences of inadequate reactive power support. It also includes details of reactive

power support available at present and efforts by planners from future perspective in

respect of NER grid.


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1 REACTIVE POWER MANAGEMENT AND VOLTAGE

CONTROL

1.1 Introduction

1.1.1 hat is Reactive Power ? Reactive power is a concept used by engineers to

describe the background energy movement in an Alternating Current

(AC) system arising from the production of electric and magnetic fields.

These fields store energy which changes through each AC cycle. Devices which

store energy by virtue of a magnetic field produced by a flow of current are said

to absorb reactive power (viz. Transformers, Reactors) and those which store

energy by virtue of electric fields are said to generate reactive power (viz.

Capacitors).

1.1.2 Power flows, both actual and potential, must be carefully controlled for a

power system to operate within acceptable voltage limits. Reactive power flows

can give rise to substantial voltage changes across the system, which means that

it is necessary to maintain reactive power balances between sources of

generation and points of demand on a 'zonal basis'. Unlike system frequency,

which is consistent throughout an interconnected system, voltages experienced

at points across the system form a "voltage profile" which is uniquely related to

local generation and demand at that instant, and is also affected by the

prevailing system network

arrangements.

1.1.3 In an interconnected AC grid, the

voltages and currents alternate up

and down 50 times per second (not

necessarily at the same time). In

that sense, these are pulsating

quantities. Because of this, the power being transmitted down a single line also

“pulsates” - although it goes up and down 100 times per second rather than 50.

1.1.4 To distinguish reactive power from real power, we use the reactive power unit

called “VAR” - which stands for Volt-Ampere-Reactive (Q). Normally electric

power is generated, transported and consumed in alternating current (AC)

W

Figure 1 Voltage and Current Waveform


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networks. Elements of AC systems supply (or produce) and consume (or absorb

or lose) two kinds of power: real power and reactive power.

1.1.5 Real power accomplishes useful work (e.g., runs motors and lights lamps).

Reactive power supports the voltages that must be controlled for system

reliability. In AC power networks, while active power corresponds to useful

work, reactive power supports voltage magnitudes that are controlled for

system reliability, voltage stability, and operational acceptability.

1.1.6 VAR Management? It is defined as the control of generator voltages,

variable transformer tap settings, compensation, switchable shunt capacitor

and reactor banks plus allocation of new shunt capacitor and reactor banks in a

manner that best achieves a reduction in system losses and/or voltage control.

1.1.7 Although active power can be transported over long distances, reactive power is

difficult to transmit, since the reactance of transmission lines is often 4 to 10

times higher than the resistance of the lines. When the transmission system is

heavily loaded, the active power losses in the

transmission system are also high. Reactive power

(vars) is required to maintain the voltage to deliver

active power (watts) through transmission lines.

When there is not enough reactive power, the voltage

sags down and it is not possible to push the power demanded by loads through

the lines. Reactive power supply is necessary in the reliable operation of AC

power systems. Several recent power outages worldwide may have been a result

of an inadequate reactive power supply which subsequently led to voltage

collapse.

1.1.8 Voltage and current may not pulsate up and down at the same time. When the

voltage and current do go up and down at the same time, only real power is

transmitted. When the voltage and current go up and down at different times,

reactive power is also gets transmitted. How much reactive power and

which direction it is flowing on a transmission line depend on how different

these two items are.

Although AC voltage and current pulsate at the same frequency, they peak at a

different time. Power is the algebraic product of voltage and current. Over a

cycle, power has an average value, called real power (P), measured in volt-

amperes, or watts. There is also a portion of power with zero average value that

is called reactive power (Q), measured in volt-amperes reactive, or vars. The

total power is called apparent power or Complex power, measured in volt-

amperes, or VA.

Figure 2 Power Triangle


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1.2 Analogy of Reactive Power

1.2.1 Why an analogy? Reactive Power is an essential aspect of the electricity system,

but one that is difficult to comprehend by a lay man. The horse and the boat

analogy best describe the Reactive Power aspect.

Visualize a boat on a canal, pulled by a horse on the bank of the canal.

The horse is not in front of the boat to do a meaningful work of pulling it in a

straight path. Due to the balancing compensation by the rudder of the boat, the

boat is made to move in a straight manner rather deviating towards the bank.

This is in line with the understanding of the reactive power.

1.2.1 In the horse and boat analogy, the horse’s objective (real power) is to move the

boat straight. The fact that the rope is being pulled from the flank of the horse

and not straight behind it, limits the horse’s capacity to deliver real work of

Figure 4 Direction of Pull

Figure 5 Vector Representation of Analogy

Figure 3 Boat Pulled by Horse


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moving straight. Therefore, the power required to keep the boat steady in

navigating straight is delivered by the rudder movement (reactive power).

Without reactive power there can be no transfer of real power, likewise without

the support of rudder, the boat cannot move in a straight line.

1.2.2 Reactive power is like the bouncing up and down that happens when we walk

on a trampoline. Because of the nature of the trampoline, that up-down

bouncing is an essential part of our forward movement across the trampoline,

even though it appears to be movement in the opposite direction.

1.2.3 Reactive power and real power work together in the way that’s illustrated very

well by the labyrinth puzzle, LABYRINTSPEL:

The description of the puzzle begins to show

why this game represents the relationship

between real and reactive power:

The intent is to manipulate a steel ball

(1.2cm in diameter) through the maze by

rotating the knobs – without letting the ball

fall into one of the holes before it reaches the

end of the maze. If a ball does fall prematurely

into a hole, a slanted floor inside the box

returns the ball to the user in the trough on the

lower right corner of the box.

1.2.4 The Objective is to twist the two knobs to adjust the angle of the platform in two

directions, in order to keep the ball rolling through the maze without falling

into any holes. Those twists are REACTIVE POWER, which helps propel the

real power through to its ultimate goal, which is delivery to the user. Without

reactive power, ball falls into holes along the way, which are NETWORK

failures.

1.2.5 Both of these examples illustrate how important it is to understand the system

and how it works in order to meet our objectives effectively. In the

LABYRINTSPEL game, if the structure of the system is not taken into account,

winning would be really easy because one knob would be turned all the way in

one direction, and the other knob all the way in the other direction, and the ball

would merely roll across the platform. If that’s the model how electricity works,

then that would deliver the electrons to the end user in the form of real power.

Figure 6 Labyrint Spel

http://gamesmuseum.uwaterloo.ca/puzzles/maze/


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But in the game, on the trampoline, and in the electric power network, the

system has more going on that means it’s essential to do things that seem

counterintuitive, like bouncing up and down on the trampoline or turning the

platform in the game towards west to avoid the hole to the east, even though we

have to go east to win.

1.2.6 In electric power, the counterintuitive thing about reactive power is to use some

power along the path to balance the flow of electrons and the circuits.

Otherwise, the electricity just flows from the generator to the largest consumer

(that’s Kirchhoff’s law, basically). In this sense, reactive power is like water

pressure in a water network.

1.2.7 LABYRINTSPEL game and the trampoline are good examples that they capture

the fact that mathematically, real power and reactive power are pure

conjugates.

1.3 Understanding Vectorally

1.3.1 In practice circuits are invariably combinations of resistance, inductance and

capacitance. The combined effect of these impedances to the flow of current is

most easily assessed by expressing the power flows as vectors that show the

angular relationship between the powers waveforms associated with each type

of impedance. Figure 7 shows how the vectors can be resolved to determine the

net capacity of the circuit needed to transfer the power requirements of the

connected equipment.

1.3.2 The useful power that can be drawn

from the electricity distribution system

is represented by the vertical vector in

the diagram and is measured in

kilowatts (kW).The reactive or wattless

power that is a consequence of the

inductive load in the circuit is

represented by the horizontal vector to

the right and the reactive power

attributable to the circuit capacitance by

the horizontal vector to the left. These

are measured in kilovars (kVAr).

Figure 7 Vector Representation


Page 11 of 59

1.3.3 The resolution of these vectors, which is the diagonal vector in the diagram is

the capacity required to transmit the active power, and is measured in kilovolts-

ampere (kVA). The ratio of the kW to kVA is the cosine of the angle in the

diagram shown as theta, and is referred to as the “power factor”.

1.3.4 When the net impedance of the circuit is solely resistance, so that the

inductance and capacitance exactly cancel each other out, then the angle theta

becomes zero and the circuit has a power factor of unity. The circuit is now

operating at its highest efficiency for transferring useful power. However, as a

net reactive power emerges the angle theta starts to increase and its cosine falls.

1.3.5 At low power factors the magnitude of the kVA vector is significantly greater

than the real power or kW vector. Since distribution assets such as cables, lines

and transformers must be sized to meet the kVA requirement, but the useful

power drawn by the customer is the kW component, a significant cost emerges

from having to over-size the distribution system to accommodate the

substantial amount of reactive power that is associated with the active power

flow.

1.4 Voltage Stability

1.4.1 Power flows, both actual and potential, must be carefully controlled for a

power system to operate within acceptable voltage limits and vice versa. Not

only is reactive power necessary to operate the transmission system reliably,

but it can also substantially improve the efficiency with which real power is

delivered to customers. Increasing reactive power production at certain

locations (usually near a load center) can sometimes alleviate transmission

constraints and allow cheaper real power to be delivered into a load pocket.

1.4.2 Voltage control (keeping voltage within defined limits) in an electric power

system is Important for proper operation of electric power equipment and

saving it from imminent damage, to reduce transmission losses and to maintain

the ability of the system to withstand disturbances and prevent voltage collapse.

In general terms, decreasing reactive power causes voltages to fall, while

increasing reactive power causes voltages to rise. A voltage collapse occurs

when the system is trying to serve much more load than the voltage can

support.


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1.4.3 As voltage drops, current must increase to maintain the power

supplied, causing the lines to consume more reactive power and the voltage to

drop further. If current increases too much, transmission lines trip, or go off-

line, overloading other lines and potentially causing cascading

failures. If voltage drops too low, some generators will automatically

disconnect to protect themselves.

1.4.4 Usually the causes of under – voltages are:

Overloading of supply transformers

Inadequate short circuit level in the point of supply

Excessive voltage drop across a long feeder

Poor power factor of the connected load

Remote system faults , while they are being cleared

Interval in re-closing of an auto-reclosure

Starting of large HP induction motors

1.4.5 If the declines continue, these voltage reductions cause additional elements to

trip, leading to further reduction in voltage and loss of load. The result is a

progressive and uncontrollable decline in voltage, all because the power system

is unable to provide the reactive power required to supply the reactive power

demand.

1.5 Voltage Collapse

1.5.1 When voltages in an area are significantly low or blackout occurs due to the

cascading events accompanying voltage instability, the problem is considered to

be a voltage collapse phenomenon. Voltage collapse normally takes place when

a power system is heavily loaded and/or has limited reactive power to support

the load. The limiting factor could be the lack of reactive power (SVC and

generators hit limits) production or the inability to transmit reactive power

through the transmission lines.

1.5.2 The main limitation in the transmission lines is the loss of large amounts of

reactive power and also line outages, which limit the transfer capacity of

reactive power through the system.

1.5.3 In the early stages of analysis, voltage collapse was viewed as a static problem

but it is now considered to be a non linear dynamic phenomenon. The dynamics


Page 13 of 59

in power systems involve the loads, and voltage stability is directly related to

the loads. Hence, voltage stability is also referred to as load stability.

1.5.4 There are other factors which also contribute to voltage collapse, and

are as below:

Increase in load

Action of tap changing transformers

Load recovery dynamics

All these factors play a significant part in voltage collapse as they effect the

transmission, consumption, and generation of reactive power.

Usually voltage stability is categorized into two parts

Large disturbance voltage stability

Small disturbance voltage stability

1.5.5 When a large disturbance occurs, the ability of the system to maintain

acceptable voltages falls due to the impact of the disturbance. Ability to

maintain voltages is dependent on the system and load characteristics, and the

Figure 8 Time frames for voltage stability phenomena


Page 14 of 59

interactions of both the continuous and the discrete controls and protections.

Similarly, the ability of the system to maintain voltages after a small

perturbation i.e. incremental change in load is referred to as small disturbance

voltage stability. It is influenced by the load characteristics, continuous control

and discrete controls at a given instant of time.

1.6 Proximity to Instability

1.6.1 Static voltage instability is mainly associated with reactive power

imbalance. Thus, the loadability of a bus in a system depends on the reactive

power support that the bus can receive from the system. As the system

approaches the maximum loading point or voltage collapse point, both real and

reactive power losses increase rapidly.

1.6.2 Therefore, the reactive power supports have to be locally adequate. With static

voltage stability, slowly developing changes in the power system occur that

eventually lead to a shortage of reactive power and declining voltage.

1.6.3 This phenomenon can be seen from a

plot of power transferred versus

voltage at the receiving end. These

plots are popularly referred to as P–V

curves or ‘Nose’ curves. As power

transfer increases, the voltage at the

receiving end decreases. In the fig(9)

eventually, a critical (nose) point, the

point at which the system reactive

power is out of usage, is reached

where any further increase in active

power transfer will lead to very rapid

decrease in voltage magnitude.

1.6.4 Before reaching the critical point, a large voltage drop due to heavy reactive

power losses is observed. The only way to save the system from voltage collapse

is to reduce the reactive power load or add additional reactive power prior to

reaching the point of voltage collapse.

Knee

point

∆v

Figure 9 PV curve and Voltage stability

margin under different conditions


Page 15 of 59

These are curves drawn between V and P of a critical bus at a constant load

power factor.

These are produced by using a series of power flow solutions for different load

levels.

At the knee point or the nose point of the V-P curve, the voltage drops rapidly

with an increase in the load demand.

Power flow solution fails to converge beyond this limit which indicates the

instability.

1.7 Reactive Reserve Margin

1.7.1 The amount of unused available capability of reactive power static as well as

dynamic in the system (at peak load for a utility system) as a percentage of total

capability is known as Reactive reserve margin.

1.7.2 Voltage collapse normally occurs when sources producing reactive power reach

their limits i.e. generators, SVCs or shunt reactors, and there is not much

reactive power to support the load. As reactive power is directly related to

voltage collapse, it can be used as a measure of voltage stability margin.

1.7.3 The voltage stability margin can be defined as a measure of how close the

system is to voltage instability, and by monitoring the reactive reserves in the

power system, proximity to voltage collapse can be monitored.

1.7.4 In case of reactive reserve criteria, the reactive power reserve of an individual or

group of VAr sources must be greater than some specified percentage (x %) of

their reactive power output under all contingencies. The precincts where

reactive power reserves were exhausted would be identified as critical areas.

1.7.5 Reactive power requirements over and above those which occur naturally are

provided by an appropriate combination of reactive source/devices which are

normally classified as static and dynamic devices.

STATIC SOURCES: Static sources are typically transmission and distribution

equipments such as Capacitors and Reactors that are relatively static and can

respond to the changes in voltage – support requirements only slowly and in

discrete steps. Devices are inexpensive, but the associated switches, control,

and communications, and their maintenance, can amount to as much as one

third of the total operations and maintenance budget of a distribution system.


Page 16 of 59

DYNAMIC SOURCES: It includes pure reactive power compensators like

synchronous condensers, Synchronous generators and solid-state devices

such as FACTS, SVC, STATCOM, D-VAR, and SuperVAR which are normally

dynamic and can respond within cycles to changing reactive power

requirement. These are typically considered as transmission service devices.

1.7.6 Static devices typically have lower capital costs than dynamic devices, and from

a system point of view, they are used to provide normal or intact-system voltage

support and to adapt to slowly changing conditions, such as daily load cycles and

scheduled transactions. By contrast, dynamic reactive power sources must be

deployed to allow the transmission system to respond to rapidly changing

conditions on the transmission system, such as sudden loss of generators or

transmission facilities. An appropriate combination of both static and dynamic

resources is needed to ensure reliable operation of the transmission system at an

appropriate level of costs.

1.7.7 Reactive power absorption occurs when current flows through an inductance.

Inductance is found in transmission lines, transformers, and induction motors

etc. The reactive power absorbed by a transmission line or transformer is

proportional to the square of the current.

Sources of Reactive Power Sinks of Reactive Power

Static:

Shunt Capacitors

Filter banks

Underground cables

Transmission lines (lightly

loaded)

Dynamic:

Synchronous

Generators/Synchronous

Condensers

FACTS (e.g.,SVC,STATCOM)

Transmission lines (Heavily

loaded)

Transformers

Shunt Reactors

Synchronous Generators

FACTS (e.g.,SVC,STATCOM)

Induction generators (wind plants)

Loads

Induction motors (Pumps, Fans

etc)

Inductive loads (Arc furnace etc)

Table 1: Reactive power compensation sources


Page 17 of 59

1.7.8 A transmission line also has capacitance. When a small amount of current is

flowing, the capacitance dominates, and the lines have a net capacitive effect

which raises voltage. This happens at night when current flows/Load is low.

During the day, when current flow/load is high, inductive effect is greater than

the capacitance, and the voltage sags.

Figure 10 Average cost of Reactive power technologies


Page 18 of 59

1.8 NER GRID – Overview

1.8.1 NER grid with a maximum peak requirement of around 2700 MW and installed

capacity of 3769 MW caters to the seven north eastern states (namely Arunachal

Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura). It is

synchronously connected with ER Grid through 400 kV BONGAIGAON – NEW

SILIGURI D/C, 400 kV BONGAIGAON – ALIPURDUAR D/C, 220 kV BIRPARA –

ALIPURDUAR D/C and internationally through 132 kV SALAKATI –

GELYPHU(Bhutan) , 132 kV RANGIA – DEOTHANG (Bhutan), 132 kV D/C

SURAJMANINAGAR – COMILLA (Bangladesh) and 11 kV MOREH –

TAMU(Myanmar). Also, it is connected to NR grid through ±800kV HVDC Bipole

Biswanath Charali-Agra link. The bottle neck of operating the NER grid arises because

of the brittle back bone network of about 8172 Ckt Kms of 132 KV lines, 3410 Ckt Kms

of 220 KV lines and 4295 Ckt Kms of 400 KV lines compared to other regional grids.

1.8.2 With Commissioning of first 800kV multi-terminal HVDC between Biswanath Charali,

Alipurduar and Agra , NER grid is directly connected with NR grid by this HVDC link.

The capacity of each terminal at Biswanath Charali(NER) and Alipurduar (ER) is 3000

MW and at Agra it is 6000 MW, at ± 800 kV voltage level.

Figure 11 NER Grid map


Page 19 of 59

1.8.3 Highlights of NER grid for current year include commercial operation of 400/220 kV

315 MVA ICT 2 at BgTPP, 132 kV Pasighat- Roing S/C line and 132 kV Roing – Tezu

S/C line. BgTPP ICT 2 commissioning shall fulfill the N-I contingency requirement of

existing 400/220 kV 315 MVA ICT at Bongaigaon. Commissioning of 132 kV Pasighat

– Roing –Teju has enabled Roing and Teju areas of Arunachal Pradesh to get

connected with the all India Grid. NTPC second unit of capacity 250 MW declared

commercially operational in the month of April. Other major elements commissioned

during current year were 20 Mvar Line Reactor in 220 kV Mariani(PG)- AGBPP at

AGBPP , 20 MVAR Bus reactor at Roing(PG), 20 MVAR Bus reactor at Teju(PG),

132/33 kV, 3x5 MVA ICT I & II at Roing (PG), 132/33 kV, 3x5 MVA ICT I & II at Teju

(PG) , 132 kV Doyang - Wokha which was LILO at Sanis, LILO of 132 kV Aizwal-

Zemabawk at Melriat(PG) and 420 kV 63 MVAR Line Reactor (to be used as Bus

Reactor) connected to 400 kV Lower Subansiri – Biswanath Chariali - I Line Bay &

400 kV Lower Subansiri – Biswanath Chariali - III Line Bay at Biswanath Chariali

(POWERGRID). Commissioning of Melriat substation has strengthened the

connectivity of Mizoram with NER grid. Wokha Substation commissioning has

improved connectivity in Nagaland and it has also provided another evacuation path

for DHEP (Doyang Hydro-Electric Power Project) of NEEPCO.

1.8.4 Almost 50% of the total NER load is spread out in 132 kV pocket of southern part of

NER which were without the direct support of major EHV trunk lines. This part of the

network was highly sensitive and was susceptible to grid disturbance in the past and

demanded more operational acumen. Increase in the loading of major 132 kV trunk

lines, in particular 132 kV DIMAPUR – IMPHAL S/C,132 kV JIRIBAM – LOKTAK S/C

and 132 kV BADARPUR – KHLIEHRIAT S/C in peak hours has led to many a grid

incidents in the past in the form of cascade tripping accompanied by voltage sag.

However, with system augmentation grid incidence in this part of the grid has become

a matter of past.

1.8.5 Relationship between frequency and voltage is a well-known fact. Studies have

revealed that though voltage is a localized factor, it is directly affected by the frequency

which is a notional factor. Any lopsidedness in the demand/generation side leading to

fluctuations in NEW grid frequency affects NER grid immensely, in particular the

voltage profile of the grid, leading to sagging and swelling of voltage heavily during

such occasions. Ironically, NER was synchronously connected with NEW grid for

stretching the transmission capability to reduce the load – generation mismatch of the

country.


Page 20 of 59

1.8.6 FSC’s have been integrated with the NER system in the 400 kV Balipara – Bongaigaon

III & IV at Balipara end.

1.8.7 Presently NER Grid is supported by 3055 MVAr from shunt reactors and 273 MVAr

from shunt capacitors spread across the region.

1.8.8 Skewness in the location of hydro stations and load centers in NER is another obstacle

which aggravates the voltage problem further. Lines are long and pass through

difficult terrains to the load centers. Northern part of NER grid which is well

supported by some strong 400 KV and 220 KV network faces high voltage regime

during lean hydro period as the corridor is not fully utilized and is usually lightly

loaded. Supports from hydro stations in condenser mode are not available for

containing low voltage conditions. D curve optimization is yet to be realized fully due

to technical glitches.

1.8.9 Reactive power management and voltage control are two aspects of a single activity

that both supports reliability and facilitates commercial transaction across

transmission network. Controlling reactive power flow can reduce losses and

congestion on the transmission system.

1.8.10 Operationally in NER, Voltage is normally controlled by managing production and

absorption of reactive power in real time :

a. By Switching in and out of Line reactance compensators such as capacitors

and shunt reactors (Line/Bus Reactors) as and when system demands in co-

operation with the constituents and the CTU.

b. By Circuit switching: Mostly one circuit of the lightly loaded d/c line is kept

open keeping in mind the n-1 criterion during high voltage and high

frequency period. Voltage differences as well as fault level of stations are

taken into account before any switching operation of circuits. Fault Level at

important substations of NER is mentioned in Annexure I.

c. By using automatic voltage regulators (AVR), the generating units control

field excitation to maintain the scheduled voltage levels at the terminals of

the generators. In real time operation, the generation/consumption of

reactive power must be within the capability curve of generator.

d. By generation re-dispatch/rescheduling.

e. By regulating voltage with the help of OLTC’s.

f. By load staggering/shedding.


Page 21 of 59

1.9 Reliability Improvement Due to Local Voltage Regulation

1.9.1 Local voltage regulation to a voltage schedule supplied by the utility can have a

very beneficial effect on overall system reliability, reducing the problems caused

by voltage dips on distribution circuits such as dimming lights, slowing or

stalling motors, dropout of contactors and solenoids, etc.

1.9.2 In past years a voltage drop would inherently reduce load, helping the situation.

Light bulbs would dim and motors would slow down with decreasing voltage.

Dimmer lights and slower motors typically draw less power, so the situation

was in a certain sense self-correcting. With modern loads, this situation is

changing.

1.9.3 Today many incandescent bulbs are being replaced with compact fluorescent

lights, LED lamps that draw constant power as voltage decreases, and motors

are being powered with adjustable-speed drives that maintain a constant speed

as voltage decreases. In addition, voltage control standards are rather

unspecific, and there is a tremendous opportunity for an improvement in

efficiency and reliability from better voltage regulation. Capacitors supply

reactive power to boost voltage, but their effect is dramatically diminished as

voltage dips.

1.9.4 Capacitor effectiveness is proportional to the square of the voltage, so at 80%

voltage, capacitors are only 64% as effective as they are at normal conditions.

As voltage continues to drop, the capacitor effect falls off until voltage collapses.

The reactive power supplied by an inverter is dynamic, it can be controlled very

rapidly, and it does not drop off with a decrease in voltage. Distribution systems

that allow customers to supply dynamic reactive power to regulate voltage

could be a tremendous asset to system reliability and efficiency by expanding

the margin to voltage collapse.


Page 22 of 59

2 TRANSMISSION LINES AND REACTIVE POWER

COMPENSATION

2.1 Introduction

2.1.1 In moving power from generators to loads, the transmission network introduces

both real and reactive losses. Housekeeping loads at substations (such as security

lighting and space conditioning) and transformer excitation losses are roughly

constant (i.e., independent of the power flows on the transmission system).

Transmission-line losses, on the other hand, depend strongly on the amount of

power being transmitted.

2.1.2 Real-power losses arise because aluminum and copper (the materials most often

used for transmission lines) are not perfect conductors; they have resistance. The

consumption of reactive power by transmission lines increases with the square of

current i.e., the transmission of reactive power requires an additional demand for

reactive power in the system components.

2.1.3 The reactive-power nature of transmission lines is associated with the geometry of

the conductors themselves (primarily the radius of the conductor) and the

geometry of the conductor configuration (the distances between each conductor

and ground and the distances among conductors).

2.1.4 The reactive-power behavior of transmission lines is complicated by their inductive

and capacitive characteristics. At low line loadings, the capacitive effect dominates,

and generators and transmission-related reactive equipment must absorb reactive

power to maintain line voltages within their appropriate limits. On the other hand,

at high line loadings, the inductive effect dominates, and generators, capacitors,

and other reactive devices must produce reactive power

2.1.5 The thermal limit is the loading point (in MVA) above which real power losses in

the equipment will overheat and damage the equipment. Most transmission

elements (e.g., conductors and transformers) have normal thermal limits below

which the equipment can operate indefinitely without any damage. These types of

equipment also have one or more emergency limits to which the equipment can be

loaded for several hours with minimal reduction in the life of the equipment.

2.1.6 If uncompensated, these line losses reduce the amount of real power that can be

transmitted from generators to loads. Transmission-line capacity decreases as the


Page 23 of 59

line length increases if there is no voltage support (injection or absorption of

reactive power) on the line.

2.2 Surge Impedance Loading (SIL)

2.2.1 Transmission lines and cables generate and consume reactive power at the

same time. The reactive power generation is almost constant, because the

voltage of the line is usually constant, and the line’s reactive power

consumption depends on the current or load connected to the line that is

variable. So at the heavy load conditions transmission lines consume reactive

power, decreasing the line voltage, and in the low load conditions – generate,

increasing line voltage.

2.2.2 The case when line’s reactive power produced by the line capacitance is equal to

the reactive power consumed by the line inductance is called natural loading or

surge impedance loading (SIL) , meaning that the line provides exactly the

amount of MVAr needed to support its voltage. The balance point at which the

inductive and capacitive effects cancel each other is typically about 40% of the

line’s thermal capacity. Lines loaded above SIL consume reactive power, while

lines loaded below SIL supply reactive power.

2.2.3 A 400 kV, line generates approximately 55 MVAR per 100 km/Ckt, when it is

idle charged due to line charging susceptance. This implies a 300 km line

generates about 165 MVAR when it is idle charged.

2.3 Shunt Compensation in Line

2.3.1 Normally there are two types of shunt reactors; Line reactor and bus reactor.

Line reactors are normally used to control over voltage due to switching and

load rejection whereas Bus reactors are normally used to control the steady

state over voltages during light load conditions.

2.3.2 The degree of compensation is decided by an economic point of view between

the capitalized cost of compensator and the capitalized cost of reactive power

from supply system over a period of time. In practice a compensator such as a

bank of capacitors (or inductors) can be divided into parallel sections, each

Switched separately, so that discrete changes


Page 24 of 59

in the compensating reactive power may be made, according to the

requirements of the load.

2.3.3 Reasons for the application of shunt capacitor units are :

Increase voltage level at the load

Improves voltage regulation if the capacitor units are properly switched.

Reduces I2R power loss in the system because of reduction in current.

Increases power factor of the source generator.

Decrease kVA loading on the source generators and circuits to relieve an

overloaded condition or release capacity for additional load growth.

By reducing kVA loading on the source generators additional kilowatt

loading may be placed on the generation if turbine capacity is available.

2.4 Line loading as function of Line Length and Compensation

2.4.1 The operating limits for transmission lines may be taken as minimum of

thermal rating of conductors and the maximum permissible line loadings

derived. SIL given in table below is for uncompensated line.

Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type


Page 25 of 59

List 1: International connectivity of NER at 400kV (Charged at 132kV)

SR.

NO

.

FROM TO UTILITY KM CKT CONDUCTOR

1 Comilla Surajmani

Nagar POWERGRID 47 1 ACSR Twin Moose

2 Comilla Surajmani

Nagar POWERGRID 47 2 ACSR Twin Moose

List 2 : International Connectivity of NER at 132kV

SR.

NO

.


1 Gelyphu(BH

U)

Salakati(IND

) POWERGRID 49.2 1 ACSR Panther

2 Motonga(BH

U) Rangia(IND) AEGCL 49 2 ACSR Panther

List 3 : +/- 800 kV HVDC Lines Agra-BNC

SR.

NO

.


1 Agra Biswanath

Charali POWERGRID 1728 1

Hexa Lapwing

2 Agra Biswanath

Charali POWERGRID 1728 2

Hexa Lapwing

List of Transmission Lines in NER GRID along with their line length and conductor type is

given in ANNEXURE II


Page 26 of 59

List 4 : Fixed, Switchable and Convertible Line reactors in North Eastern Region

SR.

NO. UTILITY FROM TO

INSTALLED

AT (STATION) KV MVAR KM CONVERTIBLE SWITCHABLE FIXED

1 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 .... …. TRUE 2 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 .... …. TRUE 3 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE …. …. 4 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE …. …. 5 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE …. …. 6 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE …. …. 7 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE …. …. 8 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE …. …. 9 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218 .... …. TRUE 10 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218 .... …. TRUE 11 POWERGRID MISA NEW MARIANI MISA 220 50 222.7 .... …. TRUE 12 POWERGRID MISA MARIANI MISA 220 50 220 …. …. TRUE 13 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247 …. TRUE …. 14 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247 …. TRUE …. 15 OTPC PALATANA SILCHAR PALLATANA 400 63 247 …. TRUE …. 16 OTPC PALATANA SILCHAR PALLATANA 400 63 247 …. TRUE ….

17 NEEPCO RANGANADI BISWANATH

CHARALI RANGANADI 400 50 204 TRUE …. ….

18 NEEPCO RANGANADI BISWANATH

CHARALI RANGANADI 400 50 204 TRUE …. ….

19 POWERGRID BISWANATH

CHARALI BALIPARA BALIPARA 400 50 65 TRUE ….

....


CHARALI BALIPARA BALIPARA 400 50 65 TRUE ….

.... 21 POWERGRID SILCHAR BYRNIHAT SILCHAR 400 63 217.14 …. TRUE ….


Page 27 of 59

22 MeECL SILCHAR BYRNIHAT BYRNIHAT 400 63 217.4 TRUE …. …. 23 POWERGRID SILCHAR AZARA SILCHAR 400 63 264 …. TRUE …. 24 AEGCL SILCHAR AZARA AZARA 400 63 264 …. …. TRUE 25 POWERGRID BANGAIGAON BYRNIHAT BANGAIGAON 400 63 167 TRUE …. …. 26 POWERGRID BANGAIGAON AZARA BANGAIGAON 400 63 118 TRUE …. ….

27 NEEPCO NEW

MARIANI AGBPP AGBPP 220 20 160.54 TRUE …. ….

NOTE: CONVERTIBLE: LINE REACTORS WHICH CAN BE OPERATED UPON ONLY WHEN LINE IS IN OUT CONDITION.

SWITCHABLE: LINE REACTORS WHICH CAN BE OPERATED EVEN WHEN LINE IS IN SERVICE.

FIXED: LINE REACTORS WHICH ARE FIXED AND CANNOT BE OPERATED UPON AS A BUS REACTOR.


Page 28 of 59

List 54: Bus Reactors in North Eastern Region

SR. NO. UTILITY INSTALLED AT

(STATION) KV

RATING STATUS

MVAR MAKE

1 POWERGRID BALIPARA 400 50 BHEL IN SERVICE

2 POWERGRID BALIPARA 400 80 BHEL IN SERVICE

3 POWERGRID BONGAIGAON 400 2 X 50 BHEL IN SERVICE

4 POWERGRID BONGAIGAON 400 2 X 80 BHEL IN SERVICE

5 POWERGRID MISA 400 50 BHEL IN SERVICE

6 POWERGRID SILCHAR 400 2 X 63 CGL IN SERVICE


CHARALI 400 2 X 80 …...

IN SERVICE

8 OTPC PALATANA 400 80 BHEL NOT IN

SERVICE

9 ASSAM MARIANI 220 2 X 12.5 .... IN SERVICE

10 ASSAM SAMAGURI 220 2 X 12.5 .... IN SERVICE

11 POWERGRID AIZWAL 132 20 .... IN SERVICE

12 POWERGRID KUMARGHAT 132 20 .... IN SERVICE

13 TRIPURA DHARMANAGAR 132 2 X 2 .... IN SERVICE

14 POWERGRID ZIRO 132 20 …. IN SERVICE

15 POWERGRID IMPHAL 132 20 …. IN SERVICE

16 POWERGRID NEW MARIANI 132 20 …. IN SERVICE

17 ASSAM SAMAGURI 132 2X12.5 …. IN SERVICE

18 ASSAM AZARA 400 63 BHEL IN SERVICE

19 MEGHALAYA BYRNIHAT 400 63 CGL NOT IN

SERVICE

20 POWERGRID ROING 132 20 …. IN SERVICE

21 POWERGRID TEJU 132 20 …. IN SERVICE


CHARALI 400 2 X 63

…. IN SERVICE


Page 29 of 59

List 6: List of Upcoming Bus Reactors in North Eastern Region

SR. NO. UTILITY

TO BE

INSTALLED AT

(STATION)

KV

RATING

(MVAR)

1 POWERGRID Namsai 132 20

2 POWERGRID Mokokchung 220 31.5

3 POWERGRID Bongaigaon 400 125

4 POWERGRID Balipara 400 125

5 AEGCL Rangia 400 80

6 POWERGRID Ranganadi 400 80

7 AEGCL Sonapur 400 80

8 POWERGRID Misa 400 80

9 POWERGRID New Mariani 400 125

10 POWERGRID Imphal 400 125

11 POWERGRID Imphal 400 80

12 POWERGRID Silchar 400 125

13 STERLITE PK Bari 400 125

14 STERLITE Surjamaninagar 400 125

List 7: Tertiary Reactors on 33kV side of 400/220/33 kV ICTs in North Eastern

Region

SR. NO. UTILITY

INSTALLED

AT

(STATION)

INSTALLED

ON

RATING

STATUS MVAR MAKE

1 POWERGRID BALIPARA 33 KV SIDE

OF ICT I 4 X 25 BHEL

IN

SERVICE

2 POWERGRID BONGAIGAON 33 KV SIDE


IN

SERVICE

3 POWERGRID MISA 33 KV SIDE


IN

SERVICE


Page 30 of 59

3 SERIES AND SHUNT CAPACITOR VOLTAGE CONTROL

3.1 Introduction

3.1.1 Capacitors aid in minimizing operating expenses and allow the utilities to serve

new loads and consumers with a minimum system investment. Series and

shunt capacitors in a power system generate reactive power to improve power

factor and voltage, thereby enhancing the system capacity and reducing the

losses.

3.1.2 In series capacitors the reactive power is proportional to the square of the load

current, thus generating reactive power when it is most needed whereas in

shunt capacitors it is proportional to the square of the voltage. Series capacitors

compensation is usually applied for long transmission lines and transient

stability improvement. Series compensation reduces net transmission line

inductive reactance. The reactive generation I2XC compensates for the reactive

consumption I2X of the transmission line. This is a self-regulating nature of

series capacitors. At light loads series capacitors have little effect.

3.1.3 This is because the

protective equipment for a

series capacitor is often

more complicated. The

factors which influence the

choice between the shunt

and series capacitors are

summarized in Table 3.

3.1.4 Due to various limitations in

the use of series capacitors,

shunt capacitors are widely

used in distribution

systems. For the same

voltage improvement, the rating

of a shunt capacitor will be higher than that of a series capacitor. Thus a series

capacitor stiffens the system, which is especially beneficial for starting large

motors from an otherwise weak power system, for reducing light flicker caused

by large fluctuating load, etc.

Table 3: Equipment preference


Page 31 of 59

3.2 MeSEB Capacity Building And Training Document Suggest (Sub

Title As Given In The PFC Document For Corporatization Of MeSEB):

3.2.1 Installation of Shunt-capacitors:

Installation of capacitors is a low cost process for reduction of technical losses.

The agricultural load mainly consists of irrigation pump motors. The PF of

pump motors are generally below 0.6, which means the total reactive power

demand of the system is high. The reactive power demand can be reduced by

installation of suitable capacitors. However, proper maintenance has to be

adopted to keep the system in order. In view of the maintenance problem,

reactive compensation technique could be installed at the distribution

transformer centers. Care has to be taken that it does not lead to over voltage

problems during the off peak hours. To avoid this there should be switch off

arrangement in the capacitor bank. The optimum allocation of LT capacitors at

distribution substation by minimizing a cost function, which includes loss cost

in the beneficiary system and the annual cost of the capacitor bank. The reactive

compensation can also be carried out at the primary distribution feeders (11

KV) lines. The optimum number, size and location of online capacitors will

depend on the following factors:

Type of load.

Quantum of load.

Load factor.

Annual load cycle.

Power factor.

3.3 As Per the Assam Gazette, Extraordinary, February 10,

2005

IN CHAPTER 9: FREQUENCY AND VOLTAGE MANAGEMENT

Sec 9.1 (d) System voltages levels can be affected by Regional operation. The SLDC shall

optimize voltage management by adjusting transformer taps to the extent

available and switching of circuits/ capacitors/ reactors and other operational

steps. SLDC will instruct generating stations to regulate MVAr generation

within their declared parameters. SLDC shall also instruct Distribution

Licensees to regulate demand, if necessary.


Page 32 of 59

List 8: Shunt Capacitors details in North Eastern Region

SR. NO. UTILITY SUBSTATION INSTALLED

ON

CAPACITY

(MVAR)

1 MeECL MAWLAI 132 KV BUS BAR 12.5

2 MeECL EPIP I 132 KV BUS BAR 20

3 MeECL EPIP II 132 KV BUS BAR 20



6 AEGCL BAGHJAB 33 KV BUS BAR 2X5

7 AEGCL KAHELIPARA 33 KV BUS BAR 3X5

8 AEGCL BARNAGAR 33 KV BUS BAR 2X5

9 AEGCL GOSAIGAON 33 KV BUS BAR 1X5

10 AEGCL GAURIPUR 33 KV BUS BAR 1X10

11 AEGCL RANGIA 33 KV BUS BAR 2X10

12 AEGCL MARGHERITA 33 KV BUS BAR 2X5

13 AEGCL N LAKHIMPUR 33 KV BUS BAR 1X5

14 AEGCL DULLAVCHERRA 33 KV BUS BAR 1X5

15 AEGCL DEPOTA 33 KV BUS BAR 2X5

16 AEGCL SARUSAJAI 33 KV BUS BAR 2X10

17 AEGCL ROWTA 33 KV BUS BAR 2X5

18 AEGCL DIPHU 33 KV BUS BAR 2X5

19 AEGCL DIBRUGARH 33 KV BUS BAR 2X10

20 AEGCL SHANKARDEV

NAGAR 33 KV BUS BAR 2X5

21 AEGCL RUPAI 33 KV BUS BAR 2X5

22 AEGCL SRIKONA 33 KV BUS BAR 2X5

Total Capacity of NER

273


Page 33 of 59

4 TRANSFORMER LOAD TAP CHANGER AND VOLTAGE

CONTROL

4.1 Introduction

4.1.1 Transformers provide the capability to raise alternating-current generation

voltages to levels that make long-distance power transfers practical and then

lowering voltages back to levels that can be distributed and used. The ratio of the

number of turns in the primary to the number of turns in the secondary coil

determines the ratio of the primary voltage to the secondary voltage. By tapping

the primary or secondary coil at various points, the ratio between the primary

and secondary voltage can be adjusted. Transformer taps can be either fixed or

adjustable under load through the use of a load-tap changer (LTC). Tap capability

is selected for each application during transformer design.

4.1.2 The OLTC alters the power

transformer turns ratio in a

number of pre-defined steps and

in that way changes the

secondary side voltage.

4.1.3 Each step usually represents a

change in LV side no-load voltage

of approximately 0.5-1.7%.

Standard tap changers offer

between ± 9 to ± 17 steps (i.e. 19

to 35 positions). The automatic

voltage regulator (AVR) is

designed to control a power

transformer with a motor driven

on-load tap-changer.

4.1.4 Typically the AVR regulates voltage at the secondary side of the power

transformer. The control method is based on a step-by-step principle which

means that a control pulse, one at a time, will be issued to the on-load tap-

changer mechanism to move it up or down by one position.

Figure 12 Switching principle of LTC


Page 34 of 59

4.1.5 The pulse is generated by the AVR whenever the measured voltage, for a given

time, deviates from the set reference value by more than the preset dead band

(i.e. degree of insensitivity). Time delay is used to avoid Unnecessary operation

during short voltage deviations from the pre-set value.

4.1.6 Transformer-tap changers can be used for voltage control, but the control differs

from that provided by reactive sources. Transformer taps can force voltage up (or

down) on one side of a transformer, but it is at the expense of reducing (or

raising) the voltage on the other side. The reactive power required to raise (or

lower) voltage on a bus is forced to flow through the transformer from the bus on

the other side.

4.1.7 The reactive power consumption of a transformer at rated current is within the

range 0.05 to 0.2 p.u. based on the transformer ratings. Fixed taps are useful

when compensating for load growth and other long-term shifts in system use.

LTCs are used for more-rapid adjustments, such as compensating for the voltage

fluctuations associated with the daily load cycle. While LTCs could potentially

provide rapid voltage control, their performance is normally intentionally

degraded. With an LTC, tap changing is accomplished by opening and closing

contacts within the transformer’s tap changing mechanism.

4.1.8 Tap optimization study is done twice in a year for obtaining the optimal tap

position of a transformer. Voltages at a particular bus, where the transformer is

connected, is plotted over a period of time in a Scatter Plot. An example of scatter

plot in given in figure 14. If the density of plots is higher in the first quadrant, i.e,

HV side voltage is higher and LV side voltage is lower, then the tap setting in

increased from the present tap setting. If the density of plots is higher in the

Third quadrant, i.e, HV side voltage is lower and LV side voltage is higher, then

the tap setting in decreased from the present tap setting. In second and fourth

quadrant, tap changing will not help in improvement of voltage.


Page 35 of 59

List of ICTs in North Eastern Region is given in ANNEXURE III.

4.2 As Per The Assam Gazette, Extraordinary, February 10, 2005

IN CHAPTER 9: FREQUENCY AND VOLTAGE MANAGEMENT

Sec 9.1(d) System voltages levels can be affected by Regional operation. The

SLDC shall optimise voltage management by adjusting transformer taps to the

extent available and switching of circuits/ capacitors/ reactors and other

operational steps. SLDC will instruct generating stations to regulate MVAr

generation within their declared parameters. SLDC shall also instruct

Distribution Licensees to regulate demand, if necessary.

Figure 13: An Example of Voltage Scatter Plot


Page 36 of 59

List 9: Transmission/Transformation/VAR Compensation Capacity of

North Eastern Region

TRANSMISSION LINE (CKT KM)

AGENCY HVDC 400 KV 220 KV 132 KV 66 KV

POWERGRID 3456 2755 1737 2568 NIL

NEEPCO NIL NIL NIL 68 NIL

NETC NIL 1328 NIL NIL NIL

ENCIL NIL 212 212 NIL NIL

STATES NIL 0 1461 5536 1492

TOTAL 3456 4295 3410 8172 1492

TRANSFORMATION CAPACITY (MVA)

POWERGRID/NEEPCO/OTPC/NHPC/

NTPC 3170/770/250/5/315 MVA

STATES 8923 MVA

TOTAL 13433 MVA

REACTIVE COMPENSATION (MVAR)

POWERGRID/NEEPCO/OTPC 2398/120/206MVAR

STATES 331 MVAR

CAPACITIVE COMPENSATION – 273 MVAR


Page 37 of 59

5 HVDC AND VOLTAGE CONTROL

5.1 Introduction

5.1.1 Basically for transferring power over a long distance or submarine power

transmission, High voltage DC transmission lines (HVDC) are preferred which

transmits power via DC (direct current). They normally consist of two converter

terminals connected by a DC transmission line and in some applications, multi-

terminal HVDC with interconnected DC transmission lines. Back-to-Back DC and

HVDC Light are specific types of HVDC systems. HVDC Light uses new cable and

converter technologies and is economical at lower power levels than traditional

HVDC.

5.2 HVDC Configuration

5.2.1 Bipolar

In bipolar transmission a pair of conductors is used, each at a high potential with

respect to ground, in opposite polarity. However, there are a number of

advantages to bipolar HVDC which can make it the attractive option.

Under normal load, negligible earth-current flows, as in the case of

monopolar transmission with a metallic earth-return. This reduces

earth return loss and environmental effects.

When a fault develops in a line, with earth return electrodes

installed at each end of the line, approximately half the rated power

can continue to flow using the earth as a return path, operating in

monopolar mode.

In very adverse terrain, the second conductor may be carried on an

independent set of transmission towers, so that some power may

continue to be transmitted even if one line is damaged.

A bipolar system may also be installed with a metallic earth return conductor. Bipolar systems may carry as much as 3000 MW at voltages of +/-800 kV (viz., 3000 MW +/- 800 KV Biswanath Charali-Alipurduar-Agra HVDC link in INDIA connecting NER GRID & ER GRID to NR GRID). Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole.


Page 38 of 59

5.2.2 Back to back

A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for

Coupling of electricity mains of different frequency (as in INDIA;

the interconnection between NEW GRID and SR GRID through

1000 MW HVDC BHADRAVATI and 1000 MW HVDC

GAZUWAKA)

Coupling two networks of the same nominal frequency but no fixed

phase relationship (viz., HVDC SASARAM, HVDC

VINDHYACHAL).

Different frequency and phase number (for example, as a

replacement for traction current converter plants)

5.2.3 A high voltage direct current (HVDC) link consists of a rectifier and an inverter.

The rectifier side of the HVDC link is equivalent to a load consuming positive real

and reactive power and the inverter side of the HVDC link as a generator

providing positive real power and negative reactive power (i.e. absorbing positive

reactive power).

5.2.4 Thyristor based HVDC converters always consume reactive power when in

operation. A DC line itself does not require reactive power and voltage drop on

the line is only the IR drop where I is the DC current. The converters at the both

ends of the line, however, draw reactive power from the AC system. The reactive

power consumption of the HVDC converter/inverter is 50-60 % of the active

power converted. It is independent of the length of the line.

5.2.5 The reactive power requirements of the converter and system have to be met by

providing appropriate reactive power in the station. For those reason reactive

power compensations devices are used together with reactive power control from

the ac side in the form of filter and capacitor banks.

5.2.6 Both AC and DC harmonics are generated in HVDC converters. AC harmonics are

injected into the AC system and DC harmonics are injected into the DC line.

These harmonics have the following harmful effects:

Interference in communication system.

http://en.wikipedia.org/wiki/Traction_current_converter_plant


Page 39 of 59

Extra power losses in machines and capacitors connected in the

system.

Some harmonics may produce resonance in AC circuits resulting in

over voltages.

Instability of converter controls.

5.2.7 Harmonics are normally minimized by using filters. The following types of filters

are used:

AC filters.

DC filters.

High frequency filters.

AC Filters

AC filters are RLC circuits connected between phase and earth. They offer low

impedance to harmonic frequencies. Thus, AC harmonic currents are passed to

earth. Both tuned and damped filter arrangements are used. The AC harmonic

AC Filter

DC Filter

DC FilterAC Filter

DC Filter

DC Filter

Converter Xmers

Valve Halls

-Thyristors

-Firing ckts

-Cooling ckt

Smoothing Reactor

Electrode station

Basic Components of HVDC TerminalBasic Components of HVDC Terminal

400 kV

DC Line

Control Room

-Control & Protection

-Telecommunication

AC PLC

Figure 12 HVDC Fundamental components


Page 40 of 59

filters also provide reactive power required for satisfactory operation of

converters and also partly injects reactive power into the system.

DC Filters

DC filters are similar to AC filters. A DC filter is connected between pole bus and

neutral bus. It diverts DC harmonics to earth and prevents them from entering

DC lines. Such a filter does not supply reactive power as DC line does not require

reactive power.

HIGH FREQUENCY FILTERS

HVDC converters may produce electrical noise in the carrier frequency band from

20 Khz to 490 Khz. They also generate radio interference noise in the mega hertz

range of frequencies. High frequency (PLC-RI) filters are used to minimize noise

and interference with PLCC. Such filters are connected between the converter

transformer and the station AC bus.

5.3 Reactive Power Source

Reactive power is required for satisfactory operation of converters and also to

boost the AC side voltages. AC harmonic filters which help in minimizing

harmonics also provide reactive power partly. Additional supply may be obtained

from shunt (switched) capacitor banks usually installed in AC side.

5.4 ±800 kV HVDC Bi-Pole

HVDC Biswanath Charali-Alipurduar-Agra is first +/- 800 kV Multi-terminal

HVDC in India with terminals located at Biswanath Charali (NER), Agra(NR) and

Alipurduar(ER) and operating at +/- 800kV. This HVDC was planned in around

2006 to evacuate generation from NER, Sikkim and Bhutan to load centres in NR

and WR. This is the first HVDC designed to evacuate power from large hydro

projects of NER and ER region. Considering the seasonality of hydro generation

this is the first HVDC bipole having flows in either direction depending upon the

season and providing flexibility and function as a pseudo phase-shifter.

The capacity of each terminal at Biswanath Charali(NER) & Alipurduar (ER) is of

3000 MW.There are two terminals at Agra with capacity of 3000MW each. After

commissioning of this HVDC it has provided first interconnection between NER

and NR and additional interconnection between ER & NR.


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5.5 Technical details of Biswanath Chariali –Alipurduar-Agra HVDC:

The Schematic Diagram of HVDC BNC-Agra is as follow.

The technical details of the line are as follows:

a. Transmission Line

a. Voltage : +/-800 kV DC

b. Length : 1726 km

c. Conductor Type : Lapwing-Hexa bundled

d. Resistance in Ohms (Approx.) : ~12.310

b. Converter Transformers

a. Agra: 4 converter transformers each with capacity of

6*295.1 MVA

b. BNC: 2 converter transformers each with capacity of

6*295.1 MVA

c. Filter Banks

a. At Agra end (Total 3705 MVAR)

Figure 13: Schematic Diagram of HVDC-BNC


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b. At BNC end (Total1983 MVAR)

S.NO Filter Bank(Z1

Capacity(MVAR)

Filter Bank(Z2

Capacity(MVAR)

Filter Bank(Z3

Capacity(MVAR)

Filter Bank(Z4

Capacity(MVAR)

Filter Bank(Z5

Capacity(MVAR)

1 Hp12 125 Hp12 125 Hp12 125 Hp12 125 Hp3 125

2 Hp12b 201 Hp12

b 201

Hp12b

201 Hp12

b 201

Hp12b

201

3 Shunt capacit

or 200

Hp24/36 b

200 Shunt capacitor

200 Hp24/36 b

200 Hp24/36

200

4 Shunt Capaci

tor 200

Shunt Capac

itor 200

Shunt Capac

itor 200

Shunt Capac

itor 200

Shunt Capac

itor 200

Table 4: AC Filter Bank at HVDC Agra

S.NO Filter

Bank(Z1 Capacity(MVAR)

Filter Bank(Z2

Capacity(MVAR)

Filter Bank(Z3

Capacity(MVAR)

1 HP12 125 HP12 125 HP12 125

2 HP12B 160 HP12B 160 HP12B 160

3 HP24/36 125 Hp24/36 125 HP24/36 125

4 HP3 159 HP24/36 125 HP3 159

5 Shunt

Capacitor 155

Shunt Capacitor

155

Table 5: AC Filter Bank at HVDC BNC.


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5.6 Impact of Largest Filter Switching Under Different HVDC Power

Order.

HVDC Power Order

Voltage at BNC before Switching of Filter

Bank

Voltage at BNC after Switching of Filter

Bank Rise in Voltage

0 417.6 440.6 23

500 426.2 453.8 27.6

1000 403.4 426.4 23

Table 6: Impact of Largest Filter Switching under different HVDC Power order.

Reactive power requirement of Converter transformer varies continuously depending

upon the power order. Reactive power generated by Switching of filters are in blocks so

at each set point there may be excess/deficit of reactive power resulting exchange of

reactive power with the grid causing bus voltage to change. Depending upon voltage to

be increased or decreased set point may be decided accordingly.


Page 44 of 59

6 FACTS AND VOLTAGE CONTROL

6.1 Introduction

6.1.1 The demands of lower power losses, faster response to system parameter change,

and higher stability of system have stimulated the development of the Flexible AC

Transmission systems (FACTS). Based on the success of research in power

electronics switching devices and advanced control technology, FACTS has

become the technology of choice in voltage control, reactive/active power flow

control, transient and steady-state stabilization that improves the operation and

functionality of existing power transmission and distribution system.

6.1.2 The achievement of these studies enlarge the efficiency of the existing generator

units, reduce the overall generation capacity and fuel consumption, and minimize

the operation cost. The power electronics-based switches in the functional blocks

of FACTS can usually be operated repeatedly and the switching time is a portion

of a periodic cycle, which is much shorter than the conventional mechanical

switches.

6.1.3 With the advanced semiconductor technology, the switching frequency and

voltage-ampere ratings of the solid switches has increased. For example, the

switching frequencies of Insulated Gate Bipolar Transistors (IGBTs) are from 3

kHz to 10 kHz which is several hundred times the utility frequency of power

system (50~60Hz).

6.2 Static Var Compensator (SVC)

6.2.1 Static Var Compensator is “a shunt-connected static Var generator or absorber

whose output is adjusted to exchange capacitive or inductive current so as to

maintain or control specific parameters of the electrical power system (typically

bus voltage)” .SVC is based on thyristors without gate turn-off capability.

6.2.2 The operating principal and characteristics of thyristors realize SVC variable

reactive impedance. SVC includes two main components and their combination:

(1) Thyristor-controlled and Thyristor-switched Reactor (TCR and TSR); and (2)

Thyristor-switched capacitor (TSC). Figure 15 shows the diagram of SVC.


Page 45 of 59

6.2.3 TCR and TSR are both composed of a shunt-connected reactor controlled by two

parallel, reverse-connected thyristors. TCR is controlled with proper firing angle

input to operate in a continuous manner, while TSR is controlled without firing angle control which results in a step change in

reactance.

6.2.4 TSC shares similar composition

and same operational mode as

TSR, but the reactor is replaced

by a capacitor. The reactance can

only be either fully connected or

fully disconnected zero due to the

characteristic of capacitor. With

different combinations of

TCR/TSR, TSC and fixed

capacitors, a SVC can meet

various requirements to

absorb/supply reactive power

from/to the transmission line.

6.3 Converter-based Compensator

6.3.1 Static Synchronous Compensator (STATCOM) is one of the key Converter-based

Compensators which are usually based on the voltage source inverter (VSI) or

current source inverter (CSI), as

shown in Figure 16 (a). Unlike SVC,

STATCOM controls the output

current independently of the AC

system voltage, while the DC side

voltage is automatically maintained

to serve as a voltage source. Mostly,

STATCOM is designed based on the

VSI (VOLTAGE SOURCE

INVERTER).

Figure 15 STATCOM topologies: (a) STATCOM based

on VSI and CSI (b) STATCOM with storage

Figure 14 Static VAR Compensators

(SVC): TCR/TSR, TSC, FC and

Mechanically Switched Resistor


Page 46 of 59

6.3.2 Compared with SVC, the topology of a STATCOM is more complicated. The

switching device of a VSI is usually a gate turn-off device paralleled by a reverse

diode; this function endows the VSI advanced controllability.

6.3.3 Various combinations of the switching devices and appropriate topology make it

possible for a STATCOM to vary the AC output voltage in both magnitude and

phase. Also, the combination of STATCOM with a different storage device or

power source (as shown in Figure 16b) endows the STATCOM the ability to

control the real power output.

6.3.4 STATCOM has much better dynamic performance than conventional reactive

power compensators like SVC. The gate turn-off ability shortens the dynamic

response time from several utility period cycles to a portion of a period cycle.

STATCOM is also much faster in improving the transient response than a SVC.

This advantage also brings higher reliability and larger operating range.

6.4 Series-connected controllers

6.4.1 As shunt-connected controllers, series- connected FACTS controllers can

also be divided into either impedance type or converter type.

6.4.2 The former includes Thyristor-

Switched Series Capacitor (TSSC),

Thyristor-Controlled Series

Capacitor (TCSC), Thyristor-

Switched Series Reactor, and

Thyristor-Controlled Series

Reactor.

6.4.3 The latter, based on VSI, is usually

in the Compensator (SSSC). The

composition and operation of

different types are similar to the

operation of the shunt connected

peers. Figure shows the diagrams

of various series-connected controllers.

Figure 16 Series-connected FACTS

controllers: (a) TCSR and TSSR; (b)

TSSC; (c) SSSC


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7 GENERATOR REACTIVE POWER AND VOLTAGE

CONTROL

7.1 Introduction

7.1.2 An electric-power generator’s primary function is to convert fuel (or other energy

resource) into electric power. Almost all generators also have considerable

control over their terminal voltage and reactive-power output.

7.1.3 The ability of a generator to

provide reactive support

depends on its real-power

production which is

represented in the form of

generator capability curve or

D - curve. Figure 18 shows the

combined limits on real and

reactive production for a

typical generator. Like most

electric equipment,

generators are limited by their

current-carrying capability.

Near rated voltage, this

capability becomes an MVA

limit for the armature of the

generator rather than a MW

limitation, shown as the

armature heating limit in the

Figure.

7.1.4 Production of reactive power involves increasing the magnetic field to raise the

generator’s terminal voltage. Increasing the magnetic field requires increasing

the current in the rotating field winding. This too is current limited, resulting in

the field-heating limit shown in the figure. Absorption of reactive power is limited

by the magnetic-flux pattern in the stator, which results in excessive heating of

the stator-end iron, the core-end heating limit. The synchronizing torque is also

reduced when absorbing large amounts of reactive power, which can also limit

Figure 17 D-Curve of a typical Generator


Page 48 of 59

generator capability to reduce the chance of losing synchronism with the system.

7.1.5 The generator prime mover (e.g., the steam turbine) is usually designed with less

capacity than the electric generator, resulting in the prime-mover limit in Fig. 18.

The designers recognize that the generator will be producing reactive power and

supporting system voltage most of the time. Providing a prime mover capable of

delivering all the mechanical power the generator can convert to electricity when

it is neither producing nor absorbing reactive power would result in

underutilization of the prime mover.

7.1.6 To produce or absorb additional VARs beyond these limits would require a

reduction in the real-power output of the unit. Capacitors supply reactive power

and have leading power factors, while inductors consume reactive power and

have lagging power factors. The convention for generators is the reverse. When

the generator is supplying reactive power, it has a lagging power factor and its

mode of operation is referred to as overexcited. When a generator consumes

reactive power, it has a leading power factor region and is under excited.

7.1.7 Control over the reactive output and the terminal voltage of the generator is

provided by adjusting the DC current in the generator’s rotating field. Control can

be automatic, continuous, and fast. The inherent characteristics of the generator

help maintain system voltage.

7.1.8 At any given field setting, the generator has a specific terminal voltage it is

attempting to hold. If the system voltage declines, the generator will inject

reactive power into the power system, tending to raise system voltage. If the

system voltage rises, the reactive output of the generator will drop, and ultimately

reactive power will flow into the generator, tending to lower system voltage.

7.1.9 The voltage regulator will accentuate this behavior by driving the field current in

the appropriate direction to obtain the desired system voltage. Because most of

the reactive limits are thermal limits associated with large pieces of equipment,

significant short-term extra reactive-power capability usually exists. Power-

system stabilizers also control generator field current and reactive-power output

in response to oscillations on the power system. This function is a part of the

network-stability ancillary service.


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7.2 Synchronous Condensers

7.2.1 Every synchronous machine (motor or generator) has the reactive power

capability. Synchronous motors are occasionally used to provide voltage support

to the power system as they provide mechanical power to their load. Some

combustion turbines and hydro units are designed to allow the generator to

operate without its mechanical power source simply to provide the reactive-

power capability to the power system when the real power generation is

unavailable or not needed.

7.2.2 Synchronous machines that are designed exclusively to provide reactive support

are called synchronous condensers. Synchronous condensers have all of the

response speed and controllability advantages of generators without the need to

construct the rest of the power plant (e.g., fuel-handling equipment and boilers).

Because they are rotating machines with moving parts and auxiliary systems,

they may require significantly more maintenance than static alternatives. They

also consume real power equal to about 3% of the machine’s reactive-power

rating. That is, a 50-MVAR synchronous condenser requires about 1.5 MW of real

power.

7.2.3 As per planning philosophy and general guidelines in the Manual on

Transmission planning criteria issued by CEA (MOP, India), Thermal / Nuclear

Generating Units shall normally not run at leading power factor. However for the

purpose of charging unit may be allowed to operate at leading power factor as per

the respective capability curve. Capability curve of various generators of NER

region are given in Annexure V.

7.2.4 Generator capability may depend significantly on the type and amount of cooling.

This is particularly true of hydrogen cooled generators where cooling gas

pressure affects both the real and reactive power capability

SL. NO. STATION UTILITY UNIT NO.

UNIT

CAPACITY

(MW)

TYPE

1 KOPILI HEP NEEPCO 1,2,3 & 4* 50 HYDEL

2 RANGANADI

HEP NEEPCO 1,2 & 3 135 HYDEL

Table 7: List of units in NER required to be normally operated with free

Governor action and AVR in service.

*Units running in 132 KV pocket is exempt from FGMO.


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8 CONCLUSION

8.1 Generators, synchronous condensers, SVCs, and STATCOMs all provide fast,

continuously controllable reactive support and voltage control. OLTC

transformers provide nearly continuous voltage control but they are slow because

the transformer moves reactive power from one bus to another, the control

gained at one bus is at the expense of the other. Capacitors and inductors are not

variable and offer control only in large steps.

8.2 An unfortunate characteristic of capacitors and capacitor-based SVCs is that

output drops dramatically when voltage is low and support is needed most. The

output of a capacitor, and the capacity of an SVC, is proportional to the square of

the terminal voltage. STATCOMs provide more support under low-voltage

conditions than capacitors or SVCs do because they are current-limited devices

and their output drops linearly with voltage.

8.3 The output of rotating machinery (i.e., generators and synchronous condensers)

rises with dropping voltage unless the field current is actively reduced.

Generators and synchronous condensers generally have additional emergency

capacity that can be used for a limited time. Voltage-control characteristics

favour the use of generators and synchronous condensers. Costs, on the other

hand, favor capacitors.

8.4 Generators have extremely high capital costs because they are designed to

produce real power, not reactive power. Even the incremental cost of obtaining

reactive support from generators is high, although it is difficult to unambiguously

separate reactive-power costs from real-power costs. Operating costs for

generators are high as well because they involve real-power losses. Finally,

because generators have other uses, they experience opportunity costs when

called upon to simultaneously provide high levels of both reactive and real power.

8.5 Synchronous condensers have the same costs as generators but, because they are

built solely to provide reactive support, their capital costs do not include the

prime mover or the balance of plant and they incur no opportunity costs. SVCs

and STATCOMs are high-cost devices, as well, although their operating costs are

lower than those for synchronous condensers and generators.


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9 SUMMARY

9.1 The process of controlling voltages and managing reactive power on

interconnected transmission systems is well understood from a technical

perspective. Three objectives dominate reactive-power management. First,

maintain adequate voltages throughout the transmission system under current

and contingency conditions. Second, minimize congestion of real-power flows.

Third, minimize real-power losses.

9.2 This process must be performed centrally because it requires a comprehensive

view of the power system to assure that control is coordinated. System operators

and planners use sophisticated computer models to design and operate the power

system reliably and economically. Central control by rule works well but may not

be the most technically and economically effective means.

9.3 The economic impact of control actions can be quite different in a

restructured/regulated industry than for vertically integrated utilities. While it

may be sufficient to measure only the response of the system in aggregate for a

vertically integrated utility, determining individual generator performance will be

critical in a competitive environment.

9.4 While it reduces or eliminates opportunity costs by providing sufficient capacity,

it can waste capital. When an investor is considering construction of new

generation, the amount of reactive capability that the generator can provide

without curtailing real-power production should depend on system requirements

and the economics of alternatives, not on a fixed rule.

9.5 The introduction of advanced devices, such as STATCOMs and SVCs, further

complicates the split between transmission- and generation based voltage

control. The fast response of these devices often allows them to substitute for

generation-based voltage control. But their high capital costs limit their use. If

these devices could participate in a competitive voltage-control market, efficient

investment would be encouraged.

9.6 In areas with high concentrations of generation, sufficient interaction among

generators is likely to allow operation of a competitive market. In other locations,

introduction of a small amount of controllable reactive support on the

transmission system might enable market provision of the bulk of the reactive

support. In other locations, existing generation would be able to exercise market

power and would continue to require economic regulation for this service.


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9.7 A determination of the extent of each type within each region would be a useful

contribution to restructuring. System planners and operators need to work

closely together during the design of new facilities and modification of existing

facilities. Planners must design adequate reactive support into the system to

provide satisfactory voltage profiles during normal and contingency operating

conditions. Of particular importance is sufficient dynamic support, such as the

reactive output of generators, which can supply additional reactive power during

contingencies.

9.8 System operators must have sufficient metering and analytical tools to be able to

tell when and if the operational reactive resources are sufficient. Operators must

remain cognizant of any equipment outages or problems that could reduce the

system’s static or dynamic reactive support below desirable levels. Ensuring that

sufficient reactive resources are available in the grid to control voltages may be

increasingly difficult because of the disintegration of the electricity industry.

9.9 Traditional vertically integrated utilities contained, within the same entity,

generator reactive resources, transmission reactive resources, and the control

center that determined what resources were needed when. Presently, these

resources and functions are placed within three different entities. In addition,

these entities have different, perhaps conflicting, goals. In particular, the owners

of generating resources will be driven, in competitive generation markets, to

maximize the earnings from their resources. They will not be willing to sacrifice

revenues from the sale of real power to produce reactive power unless

appropriately compensated.

9.10 Similarly, transmission owners will want to be sure that any costs they incur to

expand the reactive capabilities on their system (e.g., additional capacitors) will

be reflected fully in the transmission rates that they are allowed to charge.

9.11 Failure to appropriately compensate those entities that provide voltage-control

services could lead to serious reliability problems and severe constraints on inter

regional links and other congested areas as TTC (Total Transfer Capability) has a

voltage limit function as a baggage with it which is directly linked to var

compensation. With dynamic ATC’s (Available Transfer capability), Var

compensation if not seriously thought of may have serious commercial

implications in time to come due to the amount of bulk power trading happening

across the country in today’s context.


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9.12 THINGS TO DO DURING HIGH VOLTAGE CONDITION:

9.12.1 Insert shunt Reactors

9.12.2 Remove shunt Capacitors

9.12.3 Close open-ended lines or remove from service all together

9.12.4 Remove lightly loaded transmission lines from service without

compromising grid security

9.12.5 Ask the generators to maximize Var absorption within their capability

curve i.e, lower the AVR set point.

9.13 THINGS TO DO DURING LOW VOLTAGE CONDITION:

9.13.1 Remove shunt Reactors

9.13.2 Insert shunt Capacitors

9.13.3 Energize open transmission lines

9.13.4 Ask the generators to maximize Var generation within their capability

curve i.e, raise the AVR set point.

9.13.5 Shed interruptible inductive loads.


Page 54 of 59

10 STATUTORY PROVISIONS FOR REACTIVE POWER

MANAGEMENT AND VOLTAGE CONTROL

10.1 Provision in the Central Electricity Authority (Technical

Standard for connectivity to the grid) Regulations 2007 [8]:

Extracts from this standard is as reproduced below for ready reference.

Part II: Grid Connectivity Standards applicable to the Generating Units The units at

a generating station proposed to be connected to the grid shall comply with the

following requirements besides the general connectivity conditions given in the

regulations and general requirements given in part-I of the Schedule:-

1. New Generating Units

Hydro generating units having rated capacity of 50 MW and above shall be capable

of operation in synchronous condenser mode, where ever feasible

2. Existing Units

For thermal generating unit having rated capacity of 200 MW and above and

hydro Units having rated capacity of 100 MW and above, the following facilities would be

provided at the time of renovation and modernization.

(1) Every generating unit shall have Automatic Voltage Regulator. Generators having rated capacity of 100 MW and above shall have Automatic Voltage Regulator

with two separate with two separate channels having independent inputs and

automatic changeover.

10.2 Provision in The Indian Electricity Grid Code (IEGC), 2010:

10. 2.1 As per sec 3.5 of IEGC planning criterion general policy

(a) The planning criterion are based on the security philosophy on which the ISTS

has been planned. The security philosophy may be as per the Transmission Planning

Criteria and other guidelines as given by CEA. The general policy shall be as detailed

below:

a) As a general rule, the ISTS shall be capable of withstanding and be secured

against the following contingency outages

a. without necessitating load shedding or rescheduling of generation during Steady

State Operation:

-Outage of a 132 kV D/C line or,


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-Outage of a 220 kV D/C line or,

-Outage of a 400 kV S/C line or,

-Outage of single Interconnecting Transformer, Or

-Outage of one pole of HVDC Bipole line, or one pole of HVDC back to back

Station or

-Outage of 765 kV S/C line.

b. without necessitating load shedding but could be with rescheduling of

generation during steady state operation-

- Outage of a 400 kV S/C line with TCSC, or

- Outage of a 400kV D/C line, or

- Outage of both pole of HVDC Bipole line or both poles of HVDC back

to back Station or

- Outage of a 765kV S/C line with series compensation.

ii) The above contingencies shall be considered assuming a pre-contingency system

depletion (Planned outage) of another 220 kV D/C line or 400 kV S/C line in another

corridor and not emanating from the same substation. The planning study would

assume that all the Generating Units may operate within their reactive capability curves

and the network voltage profile shall also be maintained within voltage limits specified

(e) CTU shall carry out planning studies for Reactive Power compensation of ISTS

including reactive power compensation requirement at the generator’s /bulk

consumer’s switchyard and for connectivity of new generator/ bulk consumer to the

ISTS in accordance with Central Electricity Regulatory Commission ( Grant of

Connectivity, Long-term Access and Medium-term Open Access in inter-state

Transmission and related matters) Regulations, 2009.

10.2.2 As per Sec 4.6.1 of IEGC, Important Technical Requirements for

Connectivity to the Grid:

Reactive Power Compensation

a) Reactive Power compensation and/or other facilities, shall be provided by STUs, and

Users connected to ISTS as far as possible in the low voltage systems close to the load

points thereby avoiding the need for exchange of Reactive Power to/from ISTS and to

maintain ISTS voltage within the specified range. b) The person already connected to the grid shall also provide additional reactive

compensation as per the quantum and time frame decided by respective RPC in

consultation with RLDC. The Users and STUs shall provide information to RPC

and RLDC regarding the installation and healthiness of the reactive

compensation equipment on regular basis.

RPC shall regularly monitor the status in this regard.


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10.2.3 In chapter 5 of IEGC operating code for regional grids:

5.2(k) All generating units shall normally have their automatic voltage regulators

(AVRs) in operation. In particular, if a generating unit of over fifty (50) MW size

is required to be operated without its AVR in service, the RLDC shall be

immediately intimated about the reason and duration, and its permission

obtained. Power System Stabilizers (PSS) in AVRs of generating units (wherever

provided), shall be got properly tuned by the respective generating unit owner as

per a plan prepared for the purpose by the CTU/RPC from time to time. CTU

/RPC will be allowed to carry out checking of PSS and further tuning it, wherever

considered necessary.

5.2(o) All Users, STU/SLDC , CTU/RLDC and NLDC, shall also facilitate identification,

installation and commissioning of System Protection Schemes (SPS) (including

inter-tripping and run-back) in the power system to operate the transmission

system closer to their limits and to protect against situations such as voltage

collapse and cascade tripping, tripping of important corridors/flow-gates etc..

Such schemes would be finalized by the concerned RPC forum, and shall always

be kept in service. If any SPS is to be taken out of service, permission of RLDC

shall be obtained indicating reason and duration of anticipated outage from

service.

5.2(s All Users, RLDC, SLDC STUs , CTU and NLDC shall take all possible measures

to ensure that the grid voltage always remains within the following operating range.

Table 8: IEGC operating voltage range

Voltage – (KV rms)

Nominal Maximum Minimum

765 800 728

400 420 380

220 245 198

132 145 122

110 121 99

66 72 60

33 36 30


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5.2(u) (ii) During the wind generator start-up, the wind generator shall ensure that the

reactive power drawl (inrush currents in case of induction generators) shall

not affect the grid performance.

10.2.4 In chapter 6 of IEGC Section-6.6 Reactive Power & Voltage Control:

1. Reactive power compensation should ideally be provided locally, by generating

reactive power as close to the reactive power consumption as possible. The Regional

Entities except Generating Stations are therefore expected to provide local VAr

compensation/generation such that they do not draw VArs from the EHV grid,

particularly under low-voltage condition. To discourage VAr drawals by Regional

Entities except Generating Stations, VAr exchanges with ISTS shall be priced as

follows:

- The Regional Entity except Generating Stations pays for VAr drawal when voltage

at the metering point is below 97% - The Regional Entity except Generating Stations gets paid for VAr return when

voltage is below 97% - The Regional Entity except Generating Stations gets paid for VAr drawal when

voltage is above103%

- The Regional Entity except Generating Stations pays for VAr return when voltage is

above 103% Provided that there shall be no charge/payment for VAr drawal/return

by a regional Entity except Generating Stations on its own line emanating directly

from an ISGS.

2. The charge for VArh shall be at the rate of 10 paise/kVArh w.e.f. 1.4.2010, and this will

be applicable between the Regional Entity, except Generating Stations, and the

regional pool account for VAr interchanges. This rate shall be escalated at

0.5paise/kVArh per year thereafter, unless otherwise revised by the Commission. 3 Notwithstanding the above, RLDC may direct a Regional Entity except Generating

Stations to curtail its VAr drawal/injection in case the security of grid or safety of any

equipment is endangered. 4. In general, the Regional Entities except Generating Stations shall endeavor to

minimize the VAr drawal at an interchange point when the voltage at that point is

below 95% of rated, and shall not return VAr when the voltage is above 105%. ICT taps

at the respective drawal points may be changed to control the VAr interchange as per a

Regional Entity except Generating Stations’s request to the RLDC, but only at

reasonable intervals.


Page 58 of 59

5. Switching in/out of all 400 kV bus and line Reactors throughout the grid shall be

carried out as per instructions of RLDC. Tap changing on all 400/220 kV ICTs shall

also be done as per RLDCs instructions only.

6. The ISGS and other generating stations connected to regional grid shall

generate/absorb reactive power as per instructions of RLDC, within capability limits

of the respective generating units, that is without sacrificing on the active generation

required at that time. No payments shall be made to the generating companies for

such VAr generation/absorption. 7. VAr exchange directly between two Regional Entities except Generating Stations on

the interconnecting lines owned by them (singly or jointly) generally address or cause

a local voltage problem, and generally do not have an impact on the voltage profile of

the regional grid. Accordingly, the management/control and commercial handling of

the VAr exchanges on such lines shall be as per following provisions, on case-by-case

basis:

i) The two concerned Regional Entities except Generating Stations may mutually

agree not to have any charge/payment for VAr exchanges between them on an

interconnecting line.

ii) The two concerned Regional Entities except Generating Stations may mutually

agree to adopt a payment rate/scheme for VAr exchanges between them identical

to or at variance from that specified by CERC for VAr exchanges with ISTS. If the

agreed scheme requires any additional metering, the same shall be arranged by the

concerned Beneficiaries.

iii) In case of a disagreement between the concerned Regional Entities except

Generating Stations (e.g. one party wanting to have the charge/payment for VAr

exchanges, and the other party refusing to have the scheme), the scheme as

specified in Annexure-2 shall be applied. The per kVArh rate shall be as specified

by CERC for VAr exchanges with ISTS.

iv) The computation and payments for such VAr exchanges shall be effected as

mutually agreed between the two Beneficiaries.


Page 59 of 59

11. BIBLIOGRAPHY:

1. Best practice manual of transformer for BEE and IREDA by Devki energy

consultancy pvt. ltd.

2. NERPC progress report August, 2010.

3. Document on MeSEB capacity building and training document

4. Manual on Transmission Planning Criteria, CEA, Govt. of India, June

1994

5. Indian Electricity Grid Code, CERC, India, 2010 with Amendment.

6. The Central Electricity Authority (Technical Standard for connectivity to

the grid) Regulations 2007.

7. Operation procedure for NER July 2017.

8. Document on Metering code for AEGCL grid.

9. Principles of efficient and reliable reactive power supply and

consumption, staff report, FERC, Docket No. AD05-1-000, February 4,

2005

10. Proceedings of workshop on grid security & management 28th and 29th

April, 2008 Bangalore.

11. Extra High Voltage AC transmission Engineering – R D Begamudre.

12. Electrical Engineering Handbook – SIEMENS.

13. C. W. Taylor, “Power System Voltage Stability”, McGraw-Hill, 1994.

14. THE AEGCL GAZETTE, EXTRAORDINARY, FEBRUARY 10, 2005

North Eastern Regional Load Despatch Centre

Shillong

Power System operation Corporation Limited (A Government of India Enterprise)

Dongtieh-Lower Nongrah –Lapalang

Shillong-793006

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